Electrical resistance unit and method of producing the same



Feli- 24, 1931- L. T. RICHARDSON El? AL 1,793,829

ELECTRICAL RESISTANCE UNIT AND METHOD OF PRODUCING THE SAME Filed Jan 2, 1925 Pa itent ed' Feb. 24, 1931 UNITED srn'res PATENT OFFICE LINWOGD '1. RICHARDSON AND EDGAR M. SCHMIEL, OF MILWAUKEE, WISCONSIN, AS-

.SIGNORS, BY MESNE ASSIGNMENTS, TO CUTLER-HAMMER, INC., A CORPORATION OF DELAWARE ELECTRICAL RESISTANCE UNIT AND METHOD OF PRODUCING THE SAME Application filed January 2, 1925. Serial No. 25.

This invention relates to enclosed electric resistors and to a method of producing the same.

While not limited thereto, this invention is particularly applicable to enclosed electric resistors, as, for example, the so-called tubular heaters comprising a resistor, preferably helical, which is located in a tubular casing or other enclosure and insulated from the walls thereof by a filling of heat refractory material. Such heaters are now in demand 7 for high duty heating and in many instances they must be curved or otherwise shaped and bent to fit the device to be heated.

For such heaters fused magnesium oxide has been used in granular and other divided forms. To uniformly fill such a hcaterwith insulating material, the material should be inserted while in granular or divided state and heretofore 1t has been found advantageous after filling theheaterto compact the insulating material by decreasing the cross 7 Such contraction of theheater wallswhile thus serving a useful purpose has distinct disadvantages among which may bev mentioned the very substantial variation of the I ohmic value of the resistor as a result of variations of the cross-section and length of the resistor, and the tendency of the insulating material to flow longitudinally of the-casing with consequent impairment of the heater.

The types of enclosures or casings which may be used with such contraction processes are also limited to the exclusion, for example, of most forms of cast metal enclosures. Moreover, heaters so constructed have been known to burst in service as the result of subsequent chemical action on the magnesium oxide brought about by the action of water, steam i or other conditions.

The present invention has among its objects the elimination of the above difiiculties heretofore experienced in the use of such i11- sulating material and yet enabling use thereof in granular or other divided forms, which,

" um oxide. section of the casing, theresulting increase 'in density being relied upon to protect the heater against impairment while being bent or shaped for its various uses.

oxide,

as before stated, are very desirable in securing uniform filling.

The present invention includes a process for treating magnesium oxide, when placed I around a resistor Within a protective enclosure, such treatment resulting in disintegration and redistribution of the insulating material which eliminates comparatively large and irregular voids and gives permanent anchorage for the resistor in the desired posi tion. The process involving this invention is applicable to the insulation of resistors within a wide variety of protective enclosures regardless of whether or not such enclosures are capable of undergoing contraction processes or other change of shape or form.

A further object of this invention is to provide an insulating material comprising magnesium oxide in such form that it is sub stantially free from the usual detrimental expansive efiects of moisture upon magnesi- Further objects and advantages will appear upon reference to the-"followin descrip-' tion and the appended claims. T 1e inven tion will now be further described, reference being had to the accompanying drawing wherein- The figure shows partly in section a tubular heater as made in accordance with this invention;

In applying the present invention to tubular heaters utilizing magnesium oxide or insulation mixtures containing magnesium a resistor may be placed within a pro tective enclosure and the remaining space in the enclosure surrounding the resistor may be filled with granular or other divided forms of the insulation by any of the suitable and well-known methods of the prior art. The magnesium oxide is then subjected to the action of a hydrating agent, the hydrating action being preferably accelerated. Such processing of the heater chemically disintegrates the particles or granules of the oxide. The identity of the original oxide particles is lost and new and relatively much smaller particles are formed and uniformly distributed during the reaction and with a consesulated therefrom by an insulating filling 3, the ends of the resistor being projected through insulating bushings 4 and bent to engage the ends of the bushings, the bushings having openings 5 therein for a purpose hereinafter set forth.

The filling may comprise fused or other divided forms of magnesium oxide or a mixture of fused magnesium oxide and a material which remains more or less inert during the processing aforedescribed, alundum being material preferably employed. Such filling in granular or divided form may be introduced into the heater in the manner disclosed in the Lightfoot Patent, No. 1,359,400, granted November 16, 1920. In accordance with such a process, briefly stated, it is pre ferred to locate the helix in the casing with the lower end of the helix fastened to the casing through the medium of a bushing, as illustrated, and then, with the helix held under a slight tension, introduce the insulating filling and tamp the same with a device which serves also to center the helix. The structure may be vibrated during filling, if desired, but tamping of the filling is ordinarily sufficient. This method is particularly advantageous in that it permits use of insulation in fairly coarse granular form, and such form of insulation facilitates wetting thereof for the processing as hereinafter set forth. On the other hand, it is probably desirable to use the smallest granules of insulation which will permit of thorough and expeditious wetting of the mass. lVhen the casing is thus filled, the open end thereof may be partially closed by a bushing, as illustrated, or it may be-left open.

Having thus filled the heater, water may be drawn into the same through openings as at 5 by any convenient method, as by immersing one end of the heater in water and attaching the other end of the heater to a Vacuum pump, or attaching one end of the heater to a source of water pressure. This operation is preferably continued until the water fills substantially all voids within the casing, and then preferably the filling is subjected to heat and pressure. A convenient method of thus treating the filling is to place the heater in an autoclave, subjecting the filling to the action of steam in the neighborhood of 330 F. or more for a suitable period of time.

Under this treatment the magnesium oxide, whether used alone or mixed with a more or less inert material, reacts and disintegrates with accompanying expansive effects. Such expansive effects give rise to danger of bursting the casing unless the magnesium oxide is mixed with a more or less inert material or unless the casing is designed or reinforced as by a sheath to withstand the expansive effects whatever the contained amount of magnesium oxide may be. As will be apparent, the inert material in nowise affects the action of the magnesium oxide employed and, whether the filling comprises magnesium oxide alone or a mixture,-the aforementioned chemical disintegration of the magnesium oxide under restraint by the casing will cause the particles thereof as they are disrupted to be deflected into the oxide of the original mass, thereby changing the character of the mass as aforedescribed. The expansion effects of the magnesium oxide may also be controlled by regulating the amount of water available' for the reaction described which converts the magnesium oxide into magnesium hydroxide. The conversion may also be controlled by varying the duration or extent of the heat treatment. Accordingly, by such methods of control, the percentage of inert material employed may be reducedor the use of inert material may be avoided.

"However, the use of the aforementioned inert substance has been found to give very satisfactory results even when used in quantities as high as sixty to seventy per cent of..the total filling.

- lVhen the magnesium oxide is mixedwith a-more or less inert material care should be exercised in obtaining and maintaining a thorough mixture, as, otherwise, the magnesium oxide may collect at one or more points within the casing in such quantity as' to subject the latter to excessive localized internal pressures during the aforedescribed rocessing. However, the following mixture lias been given very satisfactory results:

Fused magnesium oxide (35%) 60 to mesh Alundum (85%) 90 to 120 mesh nesium hydroxide to magnesium oxide. This may be conveniently accomplished by heating of the devicein a furnace, thus at'the same time also permittingthe casing to be annealed.

While in such final treatment of the filling its weight is restored to normal, its apparent volume remains the same as when the magnesium compound is in the hydroxide state, and the heating device may be subjected to vibration without displacement of the particles. Heaters constructed as described have proven very satisfactory in service even when operated at high temperatures. Moreover, such heaters may be shaped and curved withoutendangering the resistor OI lI1 any wise impairing the condition of the heater for its various intended uses.

As will be apparent, the heater may be formed as described without subjecting the resistor to pressure which will change the cross-section thereof. Moreover, after such processing, the filling has been found to be substantially immune to further expansive effects of Water or steam.

After processing as aforedescribed, or at any other appropriate time, the bushings a may be removed and any desirable form of terminals as disclosed in the prior art may be added to the structure.

It is, of course, to be understood that any other material capable of use in the manner and for the purpose described may be sub stituted for the magnesium oxide and that fluids other than water or steam which are known to be capable of hydrating the oxide or its equivalent may be used. The invention is also applicable to forms of electrical conductors and heaters other than those specifically described. Furthermore, it will be understood that if expansion of the enclosures is not objectionable, the precautions herein setforth need not be taken.

That we claim as new and desire to obtain by Letters Patent is:

1. The process of insulating an electrical resistor which comprises surrounding the resistor with an enclosure having its walls spaced from the resistor, then placing an insulating filling comprising granular or divided magnesium oxide in the space around the resistor, then subjecting the magnesium oxide to conditions adapted to bring about its successive hydration and dehydration.

2. The process of embedding an electrical resistor in insulation which comprises surrounding the resistor with an enclosure having its walls spaced from the resistor, then placing in the space around the resistor a material comprising loose granular magnesium oxide, then hydrating the oxide in situ by fluid means admitted to the enclosure, and then heating the enclosure to expel such fiuid.

3. The process of constructing an enclosed resistor which comprises placing the resistor Within its casing, introducing into the casing in loose granular or divided form a mixture of fused magnesium oxide and more or less inert insulating material to surround the resistor, and subjecting the magnesium oxide to chemical actions for its successive hydration and dehydration.

4. The process of constructing an enclosed resistor which comprises placing the resistor within its casing, spacing the resistor from the casing with a heat refractory insulating material containing granular or divided magnesium oxide, converting the oxide into hydroxide and then effecting reconversion thereof to an oxide state.

5. The process of constructing a resistance unit which comprises placing a resistor in an enclosing casing, introducing into the casing in divided form an insulating filling containing a material in oxide state, wetting the filling to convert such material to hydroxide state, subjecting the filling to treatment to expedite such conversion and then reconverting the material into an oxide state.

6. A resistance unit comprising a resistor element encased in a coherent mass of insulating material comprising a product of magnesium oxide hydrated and dehydrated in situ.

7. An electrical resistance unit comprising a resistor, a casing therefor, and a heat refractory electric insulating filling for said casing having said resistor embedded therein, said filling being non-cementitious but being set under a pressure chemically generated in said filling in situ.

8. A method of making a tubular heater having a heating coil and an enclosing casing that comprises placing expansively hydratable material within the coil and hydrating said material.

9. The method of making a tubular heater having a heating coil and an enclosing casing which comprises placing expansively hydratable material both within and without the coil and hydrating said material.

10. The method of making a tubular heater having a heating coil and an enclosing casing which. comprises placing expansively hy dratable material both within and without the coil, then hydrating said material, and thereafter transforming said hydrated material to an oxide state.

11. An electrical resistance unit comprising a resistor, a casing therefor, and a heat refractory electric insulating filling for said casing having said resistor embedded therein, said filling being non-cementitious but being set under a pressure chemically generated in said filling in situ after being disintegrated in situ by its chemical treatment.

12. An electrical resistance unit comprising a resistor, a casing therefor, and a finely divided heat refractory electric insulating filling for said casing having said resistor embedded therein. said filling being non-cementitious but being set under a pressure chemically generated in said filling in situ.

13. An electrical resistance unit comprising a tortuous resistor, a casing therefor, and a heat refractory electric insulating filling for said casing having said resistor embedded therein, said filling being non-cementitious but being set under a pressure chemically generated in said filling in situ, said resistor upon completion of said unit having its original ohmic value.

'In witness whereof, we have hereunto subscribed our names.

LINWOOD T. RICHARDSON. EDGAR M. SCHMIEL. 

